组合模式

模式动机

组合模式统一对象和对象集，使得使用相同接口使用对象和对象集。

组合模式常用于树状结构，用于统一叶子节点和树节点的访问，并且可以用于应用某一操作到所有子节点。

Go语言实现

composite.go

package composite
import "fmt"
type Component interface {
    Parent() Component
    SetParent(Component)
    Name() string
    SetName(string)
    AddChild(Component)
    Print(string)
}
const (
    LeafNode = iota
    CompositeNode
)
func NewComponent(kind int, name string) Component {
    var c Component
    switch kind {
    case LeafNode:
        c = NewLeaf()
    case CompositeNode:
        c = NewComposite()
    }
    c.SetName(name)
    return c
}
type component struct {
    parent Component
    name   string
}
func (c *component) Parent() Component {
    return c.parent
}
func (c *component) SetParent(parent Component) {
    c.parent = parent
}
func (c *component) Name() string {
    return c.name
}
func (c *component) SetName(name string) {
    c.name = name
}
func (c *component) AddChild(Component) {}
func (c *component) Print(string) {}
type Leaf struct {
    component
}
func NewLeaf() *Leaf {
    return &Leaf{}
}
func (c *Leaf) Print(pre string) {
    fmt.Printf("%s-%s\n", pre, c.Name())
}
type Composite struct {
    component
    childs []Component
}
func NewComposite() *Composite {
    return &Composite{
        childs: make([]Component, 0),
    }
}
func (c *Composite) AddChild(child Component) {
    child.SetParent(c)
    c.childs = append(c.childs, child)
}
func (c *Composite) Print(pre string) {
    fmt.Printf("%s+%s\n", pre, c.Name())
    pre += " "
    for _, comp := range c.childs {
        comp.Print(pre)
    }
}

composite_test.go

package composite
func ExampleComposite() {
    root := NewComponent(CompositeNode, "root")
    c1 := NewComponent(CompositeNode, "c1")
    c2 := NewComponent(CompositeNode, "c2")
    c3 := NewComponent(CompositeNode, "c3")
    l1 := NewComponent(LeafNode, "l1")
    l2 := NewComponent(LeafNode, "l2")
    l3 := NewComponent(LeafNode, "l3")
    root.AddChild(c1)
    root.AddChild(c2)
    c1.AddChild(c3)
    c1.AddChild(l1)
    c2.AddChild(l2)
    c2.AddChild(l3)
    root.Print("")
    // Output:
    // +root
    //  +c1
    //   +c3
    //   -l1
    //  +c2
    //   -l2
    //   -l3
}